This invention relates to the pulping and grading of waste material and more particularly to the pulping and grading of waste stock intended for use in paper or board making machines.
In one known type apparatus for pulping and grading waste material, the stock to be so pulped and graded is introduced into a generally cylindrical chamber through a tangential inlet in the radially outer periphery. An impeller provides for pulping and rotational movement of the stock within the chamber. Accepts pass through a screen in a wall behind the impeller while heavy rejects are removed from an outlet in the periphery of the chamber. Lightweight rejects are tapped or bled off the chamber from a location at the core or axial center of the chamber.
One such apparatus of this type is disclosed in U.S. Pat. No. 2,220,676. In this apparatus, there are two impellers located at opposite ends of the chamber and two perforate screens disposed, one each behind the respective impellers. A similar type unit but utilizing only one impeller at one end of the chamber is disclosed in U.S. Pat. No. 3,844,488.
The rotational movement of the stock within the chamber of a pulper grader of this type causes large centrifugal forces which result in a significant increase in the pressure near the outer periphery or cylindrical wall. As a result of the inlet line being located at the outer periphery of the chamber, a significantly large pressure head is required to force the stock into the chamber for processing.
In our patent application, U.S. Ser. No. 756,357, filed Jan. 3, 1977, now abandoned, we disclosed a concept in which the material to be pulped or graded was introduced at the axial center or core of the chamber opposite the impeller and the lightweight rejects were bled off or tapped off the chamber from an area in the peripheral wall adjacent the end wall containing the impeller. That application disclosed that such pulper separators acted somewhat in the nature of a low power, heavy duty, defibering screen rather than a vortex separator. As a result of this, it was advantageous to bleed the lightweight rejects off the chamber through the peripheral wall in an area adjacent the wall containing the perforate screen. This was an advantageous point to remove lightweight rejects since the stock had just recently passed over the screen and accepts had been removed, thus resulting in a high concentration of lightweight rejects at that point.
Extremely low specific gravity impurities, such as styrofoam or the like having a specific gravity of about 0.15 or less, do tend to migrate to and concentrate at the central core of the chamber. These extremely lightweight rejects, therefore, sometimes are not as efficiently removed from the pulper grader disclosed in our prior U.S. application, Ser. No. 756,357 filed Jan. 3, 1977. There is a need, therefore, to provide a pulper grader with low inlet pressure requirements which is also capable of more efficiently removing extremely lightweight impurities.